1. Field of the Invention
The present invention relates to an imaging apparatus and an exposure control method for the same, which adjust a position of an ND (Neutral Density) filter against an aperture for carrying out exposure control.
2. Description of the Related Art
An imaging apparatus using a solid state image sensor, such as a digital video camera or a digital still camera, carries out exposure control by adjustment of an aperture area for an imaging lens, adjustment of a position of an ND filter against an aperture, adjustment of shutter speed in opening and closing a light receiving surface of the solid state image sensor, adjustment of gain for a light intensity signal obtained from the solid state image sensor, and the like.
In the exposure control of the imaging apparatus, there is sometimes a case in which the ND filter has a state of being partially applied to the aperture (hereinafter, called “partial application state of an ND filter”). In the partial application state of the ND filter, a part of a light flux passing through the aperture is transmitted through the ND filter and the remaining part of the light flux is directly transmitted through air. Thereby, an image-forming point becomes different between the light flux transmitted through the ND filter and the light flux transmitted not through the ND filter but through only the air, and this degrades resolution of an optical object image.
The resolution of the optical object image is degraded also by light diffraction at an edge part of the ND filter. Further, the resolution of the optical object image is degraded also when at least two regions of the ND filter, which are different in optical density, are applied to the aperture. In the description hereinafter, the partial application state of the ND filter includes also a state that at least two regions of the ND filter, which are different in the optical density, are applied to the above aperture.
There is proposed an exposure control method to improve the resolution degradation of the optical object image which occurs in this partial application state of the ND filter (Japanese Patent Application Laid-Open Publication No. 2007-25432).
This exposure control method carries out the exposure control such that an exposure amount according to brightness of an object changes in a smooth straight line, when the state that the ND filter is not applied at all to the aperture (hereinafter, called “fully open state of the ND filter”) is changed to the state that the ND filter is applied completely over the aperture (hereinafter, called “fully closed state of the ND filter”) via the partial application state of the ND filter. However, this exposure control method carries out the exposure control even in the partial application state of the ND filter, and still has a problem of degrading the resolution of the optical object image.
The inventors of the present invention have verified the resolution degradation of the optical object image in the partial application state of the ND filter for the above exposure control method. FIG. 1 shows a relationship between the brightness of the object and the resolution (MTF: Modulation Transfer Function) in the above exposure control method. “vertical” shown in FIG. 1 indicates a direction of inserting the ND filter which is the same direction as that of opening or closing the aperture and also indicates a direction vertical to an optical axis of the imaging lens. Further, “horizontal” shown in FIG. 1 indicates a direction perpendicular to each of the optical axis direction of the imaging lens and the above “vertical” direction.
In addition, “60 lines/mm” shown in FIG. 1 is a resolution representing that 60 black-and-white stripe pairs can be resolved and drawn per 1 mm in the vertical or horizontal direction, and “150 lines/mm” is a resolution representing that 150 black-and white stripe pairs can be resolved and drawn per 1 mm in the vertical or horizontal direction. Further, “Aa” shown in FIG. 1 indicates the fully open state of the ND filter, “Ab” indicates the partial application state of the ND filter, and “Ac” indicates the fully closed state of the ND filter.
As shown in FIG. 1, in a range from “dark” to “bright” in the brightness of the object, while the vertical resolution changes in a large amount depending on the position change of the ND filter against the aperture, the horizontal resolution receives little influence from the position change of the ND filter against the aperture.